One of the key factors in successful placement and acceptance of an orthopaedic implant is the osseointegration between a load-bearing implant and the existing patient bone.
Osseointegration involves the formation of a direct interface between an implant and bone, without intervening soft tissue, resulting in a firmly positioned implant that is unlikely to cause any pain or problem to the patient.
In order for effective osseointegration to occur, the outer surface of the implant should contact or nearly contact the surrounding bone tissue in order to encourage the formation of new bone against or preferably within the outer surface of the implant. Any voids or depressions in the patient bone increase the chance of spaces being present between the implant and the surrounding bone, which makes it difficult for osseointegration to occur. This can lead to weakening of the implant and discourage bone growth, as stress is unable to be transferred to the surrounding bone due to the void. As placing a bone under stress promotes growth, such a situation is not optimal.
Patient-specific implants that are designed to contour exactly to a patient's anatomy are known in the art, and are used to optimise the fit of an implant into a patient. These implants are designed and generated using patient information derived from scans, then manufactured into an implant, typically using additive manufacturing.
While these implants go some way to addressing the difficulties associated with curves and depressions within a patients anatomy, in some circumstances it is not possible to shape a patient-specific implant to fill voids, as having protrusions on the implant surface introduces difficulties with locating the implant correctly in the bone. Protrusions can prevent an implant from reaching the desired position, particularly in an area where the void for the implant tapers, for example in the case of a femoral stem implant or in the replacement of the head or stem of any long bone within the body.